The present invention relates to an organic light-emitting element and an organic light-emitting display using the same.
With the advent of a full-fledged multimedia age, growing attention is being focused on a flat-panel display as a man-machine interface.
A liquid crystal display has been used as the flat-panel display. The liquid crystal display, however, has problems such as narrow angles of visibility and low responses.
In recent years, an organic electroluminescent display is being spotlighted as a next-generation flat-panel display. This light-emitting display has excellent characteristics, such as self-luminescence, wide angle of visibility and fast response characteristics.
Conventional organic light-emitting devices have the following structure. On a glass substrate are formed a first transparent electrode such as ITO, a light-emitting layer such as an organic hole transport layer, an organic light-emitting layer and an organic electron transport layer, and a second electrode with a low work function.
A voltage of a few volts is applied between the electrodes to inject holes and electrons into the respective electrodes, which, through the transport layers, combine to produce excitons. When they return to their ground states, the excitons produce light. The emitted light passes through the first transparent electrode and then is extracted from the substrate side.
Refractive indices of the organic layers, the first transparent electrode and the glass substrate are around 1.6, 2.0 and 1.5, respectively. Hence, there are critical angles at boundary surfaces between the first transparent electrode and the glass substrate and between the glass substrate and an air layer into which the outgoing light enters. The critical angle refers to an angle between the incident light and the normal of the substrate at and beyond which the light does not pass the boundary at all and a total reflection occurs. Hence, in the conventional structure, the light extraction efficiency is low and only about 20% of the produced light can be extracted.
There are two types of organic electroluminescent displays, a simple matrix display and an active matrix display.
The simple matrix display has organic layers, such as a hole transport layer, a light-emitting layer and an electron transport layer, formed at intersections between a plurality of anode lines and cathode lines. Each of the pixels lights up for only a selection time in one frame period. The simple matrix displays have an advantage of being simple in construction.
When the number of pixels increases, however, the selection time decreases. This requires increasing a drive voltage to increase the instantaneous brightness, which in turn causes another problem of reducing the longevity of the organic electroluminescent devices. Further, since the organic electroluminescent devices are current-driven, wiring resistances cause voltage drops in a large display-screen, making it difficult to realize uniform qualities among pixels. It follows therefore that there is a limit in the simple matrix displays to the ability to increase the resolution and the size of screen.
In the active matrix displays, each organic electroluminescent device that forms a pixel is connected with a drive cell made up of between two and four thin-film transistor switching devices and thus can light up for a full period of one frame. Thus there is no need to increase the brightness, which in turn extends the lives of the organic electroluminescent devices.
As described earlier, the conventional organic electroluminescent devices extract emitted light from the substrate side and thus the aperture ratio is limited by the drive cells. To solve this problem, it has been attempted to make the upper electrode transparent and extract the emission from the upper electrode side.
U.S. Pat. No. 5,703,436 discloses an organic electroluminescent device in which the upper electrode is of a two-layered structure, with the first layer used as an injection layer of Mg or Ag and the second layer using a transparent electrode of, for example, ITO (Indium Tin Oxide) and with the emitted light extracted from the upper electrode.
JP-A-6-163158 discloses an organic electroluminescent device comprising an electron injection layer made of a transparent alkaline earth metal oxide and a transparent cathode material.